Digital video consumes large amounts of storage and transmission capacity. A typical raw digital video sequence includes 15 or 30 pictures per second. Each picture can include tens or hundreds of thousands of pixels (also called pels). Each pixel represents a tiny element of the picture. In raw form, a computer commonly represents a pixel with 24 bits or more. Thus, the number of bits per second, or bit rate, of a typical raw digital video sequence can be 5 million bits/second or more.
Most computers and computer networks lack the resources to process raw digital video. For this reason, engineers use compression (also called coding or encoding) to reduce the bit rate of digital video. Compression can be lossless, in which quality of the video does not suffer but decreases in bit rate are limited by the complexity of the video. Or, compression can be lossy, in which quality of the video suffers but decreases in bit rate are more dramatic. Decompression reverses compression.
In general, video compression techniques include “intra” compression and “inter” or predictive compression. Intra compression techniques compress individual pictures, typically called I-frames or key frames. Inter compression techniques compress frames with reference to preceding and/or following frames, and inter-compressed frames are typically called predicted frames, P-frames, or B-frames.